Comparison of thermal diffusion and interfacial reactions for bulk and sputtered titanium on 316L stainless steel

Abstract

As a first step to devise a hybrid process for the production of TiC wear coatings on 316L, consisting of magnetron sputtering followed by titanium carburization, interfacial reactivity between stainless steel and titanium has to be finely understood. Systematic comparisons were carried out on diffusion couples of increasing chemical and geometrical complexity (Fe/Ti, 316L/Ti, and 316L/sputtered Ti), highlighting the formation mechanisms of interfacial structures. Transmission and scanning electron microscopy composition profiles revealed that long-range microstructures in titanium are the result of iron diffusion and oxygen impurities interactions. FeTi and Fe2Ti intermetallics formation is first kinetically driven, then favors thermodynamic stability, leading to compositional changes during thermal cycles. Their growth is shown to be non-diffusion controlled. These compounds act as diffusion barriers for chromium, and traps for carbon, indirectly generating a complex layered structure at the interface. Differences between bulk and sputtered titanium are exclusively linked to the latter smaller scale, including destabilized diffusion fronts, and superficial TiO formation by oxygen rejection after iron diffusion.